re: "Under what conditions should I make payment or refuse to make payment? If I am unsatisfied with the deliverable, should I pay anyway?"
If you hired a freelancer to do "Task X," and they then completed "Task X", then you pay them.
If you are unsatisfied with the deliverable, then you may show your dissatisfaction in two key ways:
- Leave honest, accurate feedback when you close the contract.
- You may choose to not hire the freelancer again.
Thanks to everyone for their high quality answers. This is exactly the guidance I wanted. The job is fixed-price. I was worried because this is my first use of Upwork, and the website is actually quite ambiguous about this point. Thanks again for all the clarifications; I think I will have no problem at all, and neither should the freelancers.